1. Field of the Invention
This invention relates to the field of secondary protective surface treatment of articles with metal surfaces and to compositions, in particular liquid solutions or dispersions with a liquid continuous phase, which are used for such treatments. The treatments herein are described as "secondary" because they follow some other previously used protective treatment and enhance the overall protection achieved, particularly when further protective coatings including an organic binder are applied over the secondary treatments according to this invention.
2. Statement of Related Art
In the art of treating metal surfaces, it is common practice to improve the corrosion resistance characteristics and bonding of subsequent coatings to the metal surface by coating the metal surface with a primary protective coating or film, most often of the type commonly known as a conversion coating. In addition, there is usually an outermost coating of one or more of the types commonly known as paint, lacquer, enamel, or the like which, except for optional pigment or other coloring matter and finely divided filler materials, consist primarily of one or more organic binder.
In order to improve the qualities of the primary coating, it is common to provide a secondary coating or treatment before the outermost coating. For phosphate conversion coatings, for example, one of the most common secondary treatments has been with solutions containing hexavalent chromium, and other types of secondary treatments are also known. However, none of the previously known secondary treatments are fully satisfactory in every application, and it is an object of this invention to provide improved secondary treatments, particularly ones with less pollution potential than the conventional treatments with hexavalent chromium.